In DSC, the court ruled that Alcatel owned the employee's thoughts that had not yet been committed to writing. (See http://www.theregister.co.uk/2002/08/12/alcatel_owns_us_empl...).
In DDB, the court found that Schlumberger (the oilfield services company) owned a baseball simulator written by an employee. (See http://www.finnegan.com/Publications/federalcircuit/FCCDetai...)
In Medsphere, the CTO of the company was sued for releasing the source code to their application as open source on Sourceforge. (See http://www.informationweek.com/medsphere-settles-lawsuit-wit...)
It may be only "corporate lawyers" who agree with the policies, but those lawyers tend to be very persuasive in front of judges.
DSC Communications Corp. v. Evan Brown is more relevant, and highlights while I will never live in Texas.... But I should have also said that most of these employment contracts are governed by State law, not Federal law so the amount of bullshit the companies are allowed to get away with is highly varies from state to state. Texas is the worse when it comes to employee protections. DSC would have been laughed out of court here.
Medsphere v. Shreeve was settled out of court so has no bearing on this discussion.
So none of these cases seem to be on point here.
There aren't many 5,000+ employee companies with OSS policies this friendly.
However my experience with corporate legal at any company is the people therein have complete lack of humanity or decency. They have sold their souls for BMW and a 6-7 figure salary.